# MPC Study Group: Oblivious Transfer Extension Part 3

This week’s zoom meetup was postponed, these are some personal study notes for malicious oblivious transfer extension [1] which we partly covered last week.

## Malicious Oblivious Transfer Extension

Last week we looked at how the semi-honest oblivious transfer protocol from [1, protocol 1] was not secure against a malicious receiver $P_r$. In particular, the protocol can be made malicious-secure if it is ensured that the receiver always uses the same $r$ in step 2(a) in protocol 1, for the computation of $u^i = t^i \oplus G(k^1_i) \oplus r$. We refer to this as checking the consistency of $r$, which is the topic of this post.

## Consistency Check of $r$

First, let us deal with a somewhat easier problem.
Instead of checking that *all* $u^i$ (or a sufficient amount of $u^i$) were created with the same $r$, we instead check that a pair $u^i$ and $u^j$ was created with the same $r$.

Before the start of the consistency check (step 3 in protocol 2 [1]):

- The sender $P_s$ knows:
- $s_i, s_j, k_i^{s_i}, k_j^{s_j}, u^i, u^j$.

- The receiver $P_r$ knows:
- $u^i, u^j, r^i, r^j, k_i^0, k_j^0, k_i^1, k_j^1$
- Note: we define $r^i$ as its value as derived from $u^i$: $r^i = u^i \oplus t^i \oplus G(k_i^1)$.

The goal is for the sender $P_s$ to check that $r^i = r^j$, such that the verification passes if $r^i = r^j$ and $P_r$ is not trying to cheat, and that the verification fails with probability $\frac{1}{2}$ if $r^i \neq r^j$ or $P_r$ is trying to cheat.

Consistency check of a pair $u^i, u^j$:

- $P_r$ computes and sends to $P_s$:
- $h_{i, j}^{0,0} = H(G(k_i^0) \oplus G(k_j^0))$
- $h_{i, j}^{0,1} = H(G(k_i^0) \oplus G(k_j^1))$
- $h_{i, j}^{1,0} = H(G(k_i^1) \oplus G(k_j^0))$
- $h_{i, j}^{1,1} = H(G(k_i^1) \oplus G(k_j^1))$

- $P_s$ performs following checks, fail if any of the checks fail:
- $h_{i, j}^{s_i, s_j} = H(G(k_i^{s_i}) \oplus G(k_j^{s_j}))$
- $h_{i, j}^{\overline{s_i}, \overline{s_j}} = H(G(k_i^{s_i}) \oplus G(k_j^{s_j}) \oplus u^i \oplus u^j)$
- $u^i \neq u^j$
- Note: $s_i$, $s_j$ are the bits of the random vector of $P_s$, and $\overline{s_i} = 1 - s_i$.

This check fails with probability $\frac{1}{2}$ if $r^i \neq r^j$ or $P_r$ is trying to cheat, otherwise it passes. To verify this we should consider various cases, here we look at one specific case and refer to the paper for other cases:

- Type 3 [1]: $r^i \neq r^j$ and two of $h_{i, j}^{a,b}$ for $a, b \in {0,1}$ are incorrect
- wlog, assume that the adversary has set the values $h_{i, j}^{0,0}$ and $h_{i, j}^{0,1}$ correctly:
- $h_{i, j}^{0,0} = H(G(k_i^0) \oplus G(k_j^0))$
- $h_{i, j}^{0,1} = H(G(k_i^0) \oplus G(k_j^1))$

- and the values for $h_{i, j}^{1,0}$ and $h_{i, j}^{1,1}$ incorrectly, a smart adversary may set these values to:
- $h_{i, j}^{1,0} = H(G(k_i^0) \oplus G(k_j^1) \oplus u^i \oplus u^j)$
- $h_{i, j}^{1,1} = H(G(k_i^0) \oplus G(k_j^0) \oplus u^i \oplus u^j)$

- The verification passes if $s_i$ = 0, and the adversary learns that $s_i = 0$.
- Because $s$ was chosen at random the verification fails with probability $\frac{1}{2}$.

- wlog, assume that the adversary has set the values $h_{i, j}^{0,0}$ and $h_{i, j}^{0,1}$ correctly:

Why does this work? Consider the four cases pf $s_i$ and $s_j$:

- $s_i = 0$, $s_j = 0$, $P_s$ checks:
- $h_{i, j}^{0, 0} = H(G(k_i^{0}) \oplus G(k_j^{0}))$
- $h_{i, j}^{0, 0}$ was set correctly by adversary, so this check passes.

- $h_{i, j}^{\overline{0}, \overline{0}} = H(G(k_i^{0}) \oplus G(k_j^{0}) \oplus u^i \oplus u^j)$
- $h_{i, j}^{1, 1} = H(G(k_i^0) \oplus G(k_j^0) \oplus u^i \oplus u^j)$ (see above), thus the check passes.

- $u^i \neq u^j$

- $h_{i, j}^{0, 0} = H(G(k_i^{0}) \oplus G(k_j^{0}))$
- $s_i = 0$, $s_j = 1$, $P_s$ checks:
- Pass, similar to case $s_i = 0$, $s_j = 0$.

- $s_i = 1$, $s_j = 0$, $P_s$ checks:
- $h_{i, j}^{1, 0} = H(G(k_i^{1}) \oplus G(k_j^{0}))$
- $P_r$ set $h_{i, j}^{1,0} = H(G(k_i^0) \oplus G(k_j^1) \oplus u^i \oplus u^j)$
- Because $r^i \neq r^j$ we know that: $G(k_i^{s_i}) \oplus G(k_j^{s_j}) \oplus u^i \oplus u^j \neq G(k_i^{\overline{s_i}}) \oplus G(k_i^{\overline{s_j}})$.
- Thus we know that $H(G(k_i^{1}) \oplus G(k_j^{0})) \neq H(G(k_i^0) \oplus G(k_j^1) \oplus u^i \oplus u^j)$ and the verification fails.
- Note:
- This is because $u^i \oplus u^j$
- $= (t^i \oplus G(k_i^1) \oplus r^i) \oplus (t^j \oplus G(k_j^1) \oplus r^j)$
- $= G(k_i^0) \oplus G(k_i^1) \oplus G(k_j^0) \oplus G(k_j^1) \oplus r^i \oplus r^j$.
- From this it is easy to see that if $r^i = r^j$ they cancel out and: $u^i \oplus u^j = G(k_i^0) \oplus G(k_i^1) \oplus G(k_j^0) \oplus G(k_j^1)$.
- Thus follows that iff $r^i = r^j$ then: $G(k_i^{s_i}) \oplus G(k_j^{s_j}) \oplus u^i \oplus u^j = G(k_i^{\overline{s_i}}) \oplus G(k_i^{\overline{s_j}})$.

- $h_{i, j}^{1, 0} = H(G(k_i^{1}) \oplus G(k_j^{0}))$
- $s_i = 1$, $s_j = 1$, $P_s$ checks:
- Fail, similar to case $s_i = 1$, $s_j = 0$

This way we can check that a pair of $u^i$, $u^j$ used a consistent $r$. The protocol 2 [1] continues by performing this check for every pair of $u^i$, $u^j$. The verification passes with probability $(\frac{1}{2})^\rho$ for $p$ cheating attempts by the adversary. Meaning, the adversary may learn $\rho$ bits of $s$ but will be caught with a probability of $(\frac{1}{2})^\rho$. The consistency check does not reveal any further information.

## Other Considerations

There are many details left out, so please check the reference [1]. The protocol requires further modifications to work with this consistency check. The number of oblivious transfers has to be increased by $\rho$ the statistical security parameter. And further, the $r$ value has to be appended with a random string $r’ = r || \tau$ to prevent a malicious sender from exploiting the consistency check. Further optimizations are possible, for example, reducing the number of consistency checks.

## References

- [1] Asharov, G., Lindell, Y., Schneider, T. and Zohner, M., 2015, April. More efficient oblivious transfer extensions with security for malicious adversaries. In Annual International Conference on the Theory and Applications of Cryptographic Techniques (pp. 673-701). Springer, Berlin, Heidelberg. URL